Eni said in a statement it had restarted production in 15 Libyan wells in the Abu Attifel field some 300 km south of Benghazi.
Production at the field is currently around 31,900 barrels of oil per day, it said.
That compares to pre-conflict daily output of around 70,000 barrels.
The company said other wells would be reactivated in the coming days to reach the volumes required to fill the pipeline connecting the field to the Zuetina terminal.
Eni was the largest foreign oil producer in Libya before the civil war and is keen to mend relations with interim government leaders, after hesitant Italian support for the uprising in its early stages.
"The foreign operators already present are starting operations as fast as possible, proving that their contracts are not at risk," a Milan-based oil industry specialist said.
France's Total said on Friday production at its Al-Jurf offshore field in Libya had restarted, earlier than expected.
Eni, in Libya since 1959, produced about 270,000 barrels of oil equivalent per day in 2010. Its oil production contracts are in force to 2042 and gas contracts to 2047.
In August Eni Chief Executive Paolo Scaroni said it would take six to 18 months to resume its oil production in Libya depending on the fields.
Libya was producing 1.6 million barrels per day of oil before the uprising against leader Muammar Qaddafi began in February. The civil war caused foreign workers to flee and some oil fields and export terminals were damaged.
Last Thursday a senior source in the National Oil Corporation said Libya's oil production is set to reach 500,000 barrels per day by early October.
